The Chrome Web Store originally allowed cryptocurrency mining extensions, as long as crypo mining was all the extension was supposed to do and the users knew exactly what they were getting into. Warning consumers is particularly important considering that mining scripts often take up a lot of a computer’s processing power.

“Unfortunately, approximately 90 percent of all extensions with mining scripts that developers have attempted to upload to Chrome Web Store failed to comply with these policies, and have been either rejected or removed from the store,” James Wagner, Google’s extensions platform product manager said in a blog post on Monday.

For example, in December, Google had to delete the “Archive Poster” extension, after Bleeping Computer reported that the Tumblr extension was actually mining cryptocurrency Monero on over 105,000 devices. Other extensions, like one disguised as a URL shortener, were also deleted over the last six months.

Cryptocurrencies like Bitcoin or Ethereum depend on thousands of computers to “mine” new digital currency as they authenticate and record transactions. The process consumes immense amounts of computing power, though, so fraudsters now are trying to use others’ machines so they can reap the rewards without paying the costs.

The scheme is called “cryptojacking,” and hijacked miners have been found in a Russian nuclear weapons lab and on government websites.

Google said it’s no longer accepting extensions with miners, and will be delisting all the existing ones by late June. Blockchain-related apps will still be allowed, Google said.

Facebook and Twitter recently took action against cryptocurrencies, by banning ads on their platforms. Google stopped allowing cryptocurrency ads in March.